When Qt creates a sub-surface window tree it creates multiple "dummy" surfaces which don't get a buffer attached. Only the last child has a mapped surface.

According to the Wayland spec this means that the sub-surface is not mapped. To quote:
"A sub-surface becomes mapped, when a non-NULL wl_buffer is applied and the parent surface is mapped."

And
"A sub-surface is hidden if the parent becomes hidden, or if a NULL wl_buffer is applied. These rules apply recursively through the tree of surfaces."

Please see the attached log from using systemsettings and the attached screenshot of how the tree of subsurfaces looks in this case.

The wl_surface 102 is a child to wl_surface 70, but wl_surface 70 never gets rendered.